Mg–Al hydrotalcites as efficient catalysts for aza-Michael addition reaction: A green protocol

Mg–Al hydrotalcite was synthesized by a co-precipitation method. We have studied the effect of calcination temperature and hydration of the calcined phases on their catalytic activity for the synthesis of pyrazolo[1,5-a]pyrimidine derivatives (aza-Michael addition product). The structure of the as-synthesized sample and the presence of the anions in the interlayer galleries of hydrotalcites, have been determined by X-ray diffraction and FTIR spectroscopy. On calcining the material at 450 °C, it was amorphous periclase phase. Re-hydration of the calcined phase resulted in the formation of hydrotalcite-like phase. Such treatment to the as-synthesized hydrotalcite significantly changed the pore structure and the BET-surface area as determined from N2 physisorption at 77 K. The as-synthesized Mg–Al-hydrotalcite catalyst was found to be the most efficient for the aza-Michael reaction relative to the activated solid catalysts tested. The high performance of this catalyst was attributed to the co-operative contribution of its acidic and basic sites. We have shown that this microwave assisted reaction provides an eco-friendly alternative to the conventional syntheses where soluble bases are used. Furthermore, the reaction was performed over a considerably shorter time scale and generated significantly higher yields than traditional methods.

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► A hydrotalcite is presented as a potential alternative to soluble base catalysts.
► An environmentally benign method was applied to replace the classical methods.
► The microwave assisted reaction is an efficient protocol for the products in short time.
► Mg–Al hydrotalcite catalyst shows higher catalytic efficiency than its activated forms.
► A proposed mechanism for the synthesis of the aza-Michael product has been suggested for the first time.